Optical and redox properties of phenyl-capped cyclohexa[c]-oligothiophenes

• Phenyl-capped cyclohexa[c]thiophenes (nCHT) have been synthesized and characterized.
• nCHT is the first model compound to poly(nCHT).
• nCHT has nice redox activity.
• nCHT has twisted conformation at the neutral state.
• DFT calculation supports the improved conjugation length at oxidized state.

A series of phenyl-capped cyclohexa[c]thiophene derivatives (nCHT-TEG, n = 2, 4, or 6) have been synthesized. nCHT-TEG are well-soluble in common organic solvents. The absorption spectra of neutral nCHT-TEG oligomers indicated a shorter effective conjugation length than conventional oligothiophenes based on the non-coplanarity of the thiophene rings. nCHT-TEG can be oxidized/reduced reversibly. The results of cyclic voltammetry and UV–Vis–NIR spectroscopy of the oxidized nCHT-TEG revealed that the effective conjugation length increases at the higher oxidation state. Density functional theory (DFT) calculations indicate that the quinoidal structure of the oxidized nCHT contributes to the improved effective conjugation length. nCHT-TEG radical cations and dications were characterized by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies, respectively. NMR results of nCHT-TEG dication revealed that 2CHT-TEG2+ have closed-shell bipolaron structure, while 4CHT-TEG2+ and 6CHT-TEG2+ are the mixture of closed-shell bipolaron and open-shell two-polaron structures.

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